Method of refining kraft wood pulp



Unite States Patent 3,068,141 METHOD OF REFINING KRAFT WOOD PULP RobertH. MacClaren and Frank L. Wells, Rochester,

N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., a corporationof New Jersey No Drawing. Filed Sept. 10, 1959, Ser. No. 839,052 6Claims. (Cl. 162-80) This invention relates to the preparation fromkraft wood pulp of a cellulose product useful as the starting materialin an acylation process which comprises treating the pulp with strongaqueous alkali containing a boron compound at moderate temperature andtreating the pulp with dilute aqueous alkali at an elevated temperaturefor the extraction of undesired materials therefrom.

At the present time, probably the most important methods of makingchemical wood pulp are the sulfite and the kraft pulping procedures. Upto now, most, if not all, of the wood pulps considered as useful asstarting materials for making cellulose esters have been sulfite pulps.Heretofore, refined kraft pulps have not been particularly susceptibleto acylation, the cellulose esters prepared therefrom having beencharacterized by high haze and opaquing properties. Hence, even thoughkraft cooking procedures are useful in the reclaiming of the cellulosefrom wood, the cellulose product has been useful only for purposes forwhich extensive refinement is not required.

One object of our invention is to provide a procedure for the refinementof kraft wood pulp by which that pulp can be readily converted intoacetylated cellulose from which dopes of good color and clarity can beobtained. Another object of our invention is a procedure for therefinement of kraft pulp which includes the use of boron compoundstherein. Another object of our invention is to provide a combination ofsteps for refining kraft pulp compatible with other pulp refining steps,whereby products of good propertie and good color may be obtained. Otherobjects of our invention will appear herein.

We have found that kraft pulp forms a product which is susceptible toalkaline refining at moderate temperatures providing the alkali solutionemployed contains a substantial percentage of boron compound, such assodium me-ta-borate, borax or boric acid, which treatment is followed orpreceded by an' extraction procedure using dilute alkali solution athigh temperatures. We have found that 'by so refining kraft pulp theproduct obtained will exhibit good activity when subjected to anacetylation procedure and the esterification mass obtained upon using sorefined kraft pulp will have good haze and color characteristicsindicating substantial esterification and a good cellulose esterproduct. If the kraft pulp being refined is a bleached pulp the desiredproduct may be obtained merely by a sequence of alkaline refining stepsin accordance with our invention. If, however, the kraft pulp to berefined is unbleached it i desirable that the alkaline refining steps inaccordance with our invention be interspersed with bleaching steps, suchas with aqueous chlorine, sodium hypochlorite, chlorine dioxide or thelike.

Our invention is adapted for refining kraft pulps prepared not only fromspruce but from other woods, suchas hemlock, southern pine, alder,maple, gumwood, or other types of wood the fibers of which have beenliberated by" cooking with a kraft cooking liquor.

In-its broadest aspects, our invention involves the subjecting of kraftpulp to a sequence which includes the steps extraction with a hot dilutealkali solution. The treatment may involve a treatment once with thiscombination or a treatment several times with a refining operation inice , which these steps are used. The treatment may involve treatmentwith strong alkali and boron compound at moderate temperature followedby treatment with dilute aqueous alkali at elevated temperature and asubsequent treatment with strong alkali and boron compound again atmoderate temperature. The alkaline refining steps may be separated bychlorination or bleach treatments. The pulp may be first chlorinated,then treated with a boron containing concentrated alkaline solution,then thickened, then given a high temperature digestion with dilutealkali, again thickened and then treated with a strong boron-compoundcontaining sodium hydroxide solution. The digestion steps can 'becarried out at atmospheric pressure or above atmospheric pressure.Continuous treatments and re-use of solutions may be resorted to inprocessing the pulp, particularly if continuous operations are thoughtto be desirable.

. In carrying out our invention the strong alkali treatment isordinarily carried out with aqueous sodium hydroxide having aconcentration within the range of approximately 812% which solutioncontains 2-5% of sodium borate, boric acid or borox (sodiumtetraborate). Thetreatment with the strong alkali is carried out at atemperature within the range of 1040 C., the pulp being agitated thereinforthe desired time. For instance, if the strong alkali treatment is tobe given once or twice, a time of l-3 hours may accomplish the desiredresult. On the other hand, if several treatments are to be given thelength of timeof each treatment may desirably be modified to the use ofshorter periods for each treatment. The pulp after (or before) it hasbeen subjected to the treatment with strong alkali and to the desiredchlorinations' or bleaching treatments, is treated with dilute aqueousalkali, the sodium hydroxide therein being within the range of /z2%, thetreatment being carried out at a chemicals with water and may be driedor it may be In the lat-' ter case, it would be desirable to remove thewater from the cellulose by displacement with acetic acid or someemployed directly for acetylation purposes.

other lower fatty acid giving a pulp wet with fatty acid in goodcondition for reaction with acetic anhydride or otheresterificationreagents.

The following examples illustrate a comparison of the procedures inaccordance with our invention with those omitting boron compounds andthe properties obtained upon acetylating the products refined therebywith those not in accordance with the invention. I v

Example 1 Unbleached kraft pulp prepared by cooking spruce H BO for-2hours at 30 C.

(b) 12% sodium hydroxide 'solution containing 3%;

- (c) 3% aqueous chlorine solution at 20 C.

(d) An aqueous solution of sodium hydroxide of 1% concentration forZhours at 100 C;

(2) An aqueous solution containing 1% sodium chlorite and 5% of aceticacid'for- 3'hours at f C.

(f) An aqueous solution of sodium hydroxide in 12% concentrationcontaining 3% of H BO for 2 hours at 30 C.

(g) 1% sodium chlorite and acetic acid in water at 85 C. for 3 hours-(h) Aqueous HCl in 0.5% concentration for 1 hour at The resultingrefined pulp was washed and dried and was acetylated in a sigma bladedjacketed mixer as follows:

3 pounds of pulp with a moisture content of 3 to 4% was presoaked for 30minutes at 105 F. with 6 pounds of glacial acetic acid containingsufficient water to adjust the moisture content of the pulp to 6%. Then10 pounds of glacial acetic acid and 4 cc. of sulfuric acid were addedand the mass was held minutes at 102 to 105 F. The temperature wasreduced to 78 F. over a period of 15 minutes and 7.8 pounds of aceticanhydride was added. The mass was cooled to 55 F. and a solution of 90cc. acetic acid and 41 cc. sulfuric acid was added and the temperaturewas increased to 102 F. over a period of 75 minutes. The cellulosedissolved in the esterification liquid. The mass was diluted with 10pounds of aqueous acetic acid to destroy any unused anhydride and theproperties of the product were determined. It was found to have a hazeof 17 cm. determined by the stainless steel wire test or of 30 p.p.m.when determined against standards made up from water and opaquematerial. The color was 300.

Example 2 The preceding sequence was repeated using sodium tetraborateinstead of boric acid in steps (b) and (f). Testing of the product gavethe following values: haze-21 cm. by the stainless steel wire test or 30p.p.m. when judged against standards made up from water and opaquematerials and a color of 300.

Example 3 (Omitting Boron Compounds) Unbleached spruce kraft pulp wastreated by the same sequence as listed in Example 1 but the boroncompounds were omitted. The haze was 6 cm. by the stainless steel wiretest or 100 p.p.m. by the standards test and the color was 500.

The refined pulp used here exhibited poor activity upon esterification.in comparison with the susceptibility to esterification of. the pulpsobtainedby the refinement methods employed in Examples 1 and 2.

A commercial acetylation cellulose made by the alkaline refinement ofspruce sulfite pulp was acetylated in a manner similar to that describedin the foregoing examples. The properties exhibited by the acetylatedpulp were as follows: Haze-10-12 cm. by the stainless steel wire test or60 p.p.m. by the standards test and a color of 250.

Example 4 Unbleached kraft pulps from other woods than spruceweretreated by a procedure embodying the applicants invention in which thepulp was treated first with chlorine and then repetitively as shown:

containing H3BO3, 30 C.,

These pulps were treated by the application of 2 sequences and by theapplication of 3 sequences respectively and the refined pulps thusobtained were acetylated bya procedure similar to that of the precedingexamples.

Number of Treatments 2 3 Wood type Haze, Color Haze, Color cm. cm.

19 185 26 115 Hemlock 7 285 15 275 15 235 33 110 12 550 17 Z45 Gumwood 7375 15 275 Comparisoncommerc' y refined Spruce sulfite pulp acetylate osimilarly 11 17 In the testing of the acetylation products the haze wasdetermined by means of a length of 16 gauge stainless steel wire, thehaze number being the length of wire which was inserted into a tube ofthe cellulose ester dope until the end of the wire became invisible,these tests being run. in an apparatus especially constructed for thistest. The measurements of haze by the water standards were carried outby comparing the solutions against standard solutions made up from waterand precipitated fullers earth in accordance with the Turbidity Standardfor Water Analysis, APHA specifications.

The color determinations were made by comparing cellulose estersolutions with color standard solutions. These are prepared from a colorstandard containing 500 p.p.m. of platinum purchased as platinic cobalt(APHA Color Standard). The solution may also be prepared as described inTechnical Methods of Analysis by Roger C. Griflin, page 673(McGraw-Hill, 1927), which preparation involves dissolving 1.245 gramsof potassium platinum chloride containing 0.500 gram of platinum and1.000 gram of crystallized cobalt chloride (CoCl .6H O) containing 0.248gram of cobalt in water and adding ml. of concentrated HCl and dilutingwith 1 liter of water. Color standards containing 50, 60, 70, 80, 90,100, 125, 150, 175, 200, 225, 250, 275, 300, 350, 400, 450, 500 and 550parts per million of platinum were prepared by diluting suitablealiquots of the standard solutions to 500 ml. with water. Thesestandards were placed in bottles of good clarity and free from flaws.The tests were, run by passing light from a uniform light source throughthe sample to be tested and through the color standards more nearlycorresponding thereto in color. When a color determination is notclearly like either of two color standards the color number isextrapolated between the two color standards most closely resembling thesample being tested.

Refined pulps in accordance with our invention are useful not only forthe preparation of cellulose acetates but for the preparation of otherlower fatty acid esters of cellulose, such as cellulose acetatepropionate, cellulose acetate butyrate, cellulose butyrate or the likeby conventional esterification procedures. The thus prepared celluloseesters are characterized by their usefulness for maleing up solutions inorganic solvents of good clarity and uniformity. These esters are usefulfor the preparation of sheeting, yarn, molded products and the like, asdescribed in the literature.

The application of various chlorine-containing bleaches to wood pulp iswell known in the pulp refining art and where bleaching steps arereferred to unlessthe conditions are given, the conditions used arethose conventionally employed in the pulp bleaching operations of thetype referred to.

The kraft pulp which is refined in accordance with our invention is tobe understood to include any wood pulp prepared from soft wood or hardwood prepared by a sulfate cooking process. This includes not onlyundercooked stock but as well that which has been given a longer time ofcooking.

It is to be understood that in the refining procedure in accordance withour invention it is desirable to follow the customary procedure ofsubjecting the pulp to a washmg treatment after each of the steps in therefining operation. It may some time be desirable to carry over thetreating material from one step to the other which option, of course, isleft to the judgment of the individual operator in performing therefining operations.

We claim:

1. A method of preparing refined kraft wood pulp useful as a startingmaterial in the preparation of cellulose ester products which comprisessubjecting kraft pulp to a series of treatments at least two of thesteps of which involves (1) treating the pulp with aqueous causticalkali of /2-2% concentration at 100-140 C., and ('2) treating withaqueous caustic alkali of 812% concentration containing 2-5% of a boroncompound selected from the group consisting of sodium meta-borate, boraxand boric acid.

2. A method of refining kraft wood pulp to render it useful as astarting material for cellulose esterification processes which comprisessubjecting the pulp in unbleached form to a series of treatmentsincluding the steps of (1) bleaching with a chlorine compound, (2)treating with aqueous caustic alkali of 8-12% concentration containing2-5% of a boron compound selected from the group consisting of sodiummeta-borate, borax and boric acid at -40 C., and (3) treating withaqueous caustic alkali of /2-2% concentration at 100-140 C.

3. A process of refining kraft pulp to adapt it for use as a startingmaterial for cellulose esterification processes which comprisessubjecting unbleached kraft pulp to a series of steps including 1)bleaching with a chlorine compound, (2) treating wtih aqueous causticalkali of 812% concentration containing 2-5% boric acid at 10-40 C., and(3) treating with aqueous caustic alkali of /2-2% concentration at100-140 C.

4. A process of refining kraft pulp to adapt it for use as a startingmaterial for cellulose esterification processes which comprisessubjecting unbleached kraft pulp to a series of steps including (1)bleaching with a chlorine compound, (2) treating with aqueous causticalkali of 12% concentration containing 2- 5% sodium tetraborate 4 6 at10-40" C., and (3) treating with aqueous caustic alkali of /z-2%concentration at -140 C.

5. A method of refining kraft pulp which comprises treatment of the pulpwith aqueous caustic alkali of 812% concentration containing 25% of aboron compound selected from the group consisting of sodium metaborate,borax and boric acid at 10-40 C. and subsequently treating the pulp withaqueous caustic alkali of /2-2% concentration at IOU- C.

6. A process of refining kraft pulp to adapt it for use as a startingmaterial for a cellulose esterification process which comprisessubjecting the pulp to the following series of steps in the order given:

Treating the pulp with 3% aqueous chlorine at 20 C.

Treating with 12% sodium hydroxide solution containing 3% boric acid for2 hours.

Treating with 3% aqueous chlorine solution at 20 C.

Treating with an aqueous solution of sodium hydroxide of 1%concentration for 2 hours at 100 C.

Treating with an aqueous solution containing 1% sodium chlorite and 5%acetic acid for 3 hours at 85 C.

Treating with an aqueous solution of sodium hydroxide in 12%concentration containing 3% of boric acid for 2 hours at 30 C.

Treating with 1% sodium chlorite and 5% acetic acid in water at 85 C.for 3 hours and treating with aqueous HCl of 0.5% concentration for 1hour at 25 C.

References Cited in the file of this patent UNITED STATES PATENTS1,868,570 Dils July 26, 1932 1,935,580 Richter Nov. 14, 1933 2,898,333Jullander Aug. 4, 1959 OTHER REFERENCES Casey: Pulp and Paper, vol. I,pages 310-321, published by Interscience Pub., New York, N.Y., 1952.

Mellor: Studies on Modified Cellulose, from TAPPI, vol. 38, No. 11,November 1955, pp. 682, 68 7,

1. A METHOD OF PREPARING REFINED KRAFT WOOD PULP USEFUL AS A STARTINGMATERIAL IN THE PREPARATION OF CELLULOSE ESTER PRODUCTS WHICH COMPRISESSUBJECTING KRAFT PULP TO A SERIES OF TREATMENTS AT LEAST TWO OF THESTEPS OF WHICH INVOLVES, (1) TREATING THE PULP WITH AQUEOUS CARUSTICALKALI OF 1/2-2% CONCENTRATION AT 100-140*C., AND (2) TREATING WITHAQUEOUS CAUSTIC ALKALI OF 8-12% CONCENTRATION CONTAINING 2-5% OF A BORONCOMPOUND SELECTED FROM THE GROUP CONSISTING OF SODIUM META-BORATE, BORAXAND BORIC ACID.